With continual progress of the optoelectronic technology, the requirement of people for displays has become more and more stringent. Compact flat displays have gradually replaced conventional bulky cathode ray tube (CRT) displays. As for the illumination light source, power-saving LEDs have become the mainstream, and the key material (phosphor) of the light source is under rigorous research. Phosphor is also called fluorescent material or light emitting material, which is excited by energy of different form to convert the absorbed energy into visible lights by means of non-thermal radiation.
Phosphor can generally be categorized into two types: organic phosphor and inorganic phosphor. The inorganic phosphor includes a light emitting center formed of a host and appropriate activators. At present, the host primarily used by manufacturers is generally a II-VI compounds such as an ionic compound formed of Ca, Sr, Ba (IIA group) or Zn, Cd, Hg (IIB group) and S, Se (VIA group). The activators are generally Mn, Cu, Ag, lanthanide-series elements, and transition metals such as Eu, Sm and rare earth elements. For instance, when gadolinium titanium oxide having semiconductor properties is added with rare earth element Eu3+, it becomes slightly fluorescent, and the emission wavelengths are 590 nm and 615 nm.
Nowadays, white-emission illumination light sources are the most appreciated emerging products all over the world. The earliest technology for manufacturing white-emission light sources is developed by Nichia, Japan to produce white light by mixing two wavelengths. In the method developed by Nichia, Japan, a layer of yttrium-aluminum-garnet (YAG) phosphor is coated on a 460 nm blue light emitting die. The YAG phosphor is excited by a blue LED to produce a 550 nm yellow light complementary to the blue light. By means of the principle of lens, the complementary white and blue lights are mixed to obtain the desired white light. This method has a lower manufacturing cost, and the power circuit structure is simpler.
Because Nichia, Japan has the patent of the above manufacturing technology in hand, most other manufacturers have plunged into the development of three wavelength light sources, which have blue, green and red phosphors that are excited by ultraviolet light emitted by an inorganic ultraviolet chip. If the ratio of the three primary color light is appropriate, the mixed light is a white light. There are numerous phosphors available, but it is necessary to take the manufacturing process and the emission strength into account.
On the other hand, a method has been proposed, which chooses a special phosphor capable of emitting white light under excitation of ultraviolet light. This special phosphor can emit white light without the need of mixing several color lights. However, there are rare kinds of this special phosphor and thus cannot meet various requirements.